1. Field of the Invention
The present invention relates, in general, to a flash memory device and, more particularly, to a method of forming a flash memory device in which the number of process steps can be reduced.
2. Discussion of Related Art
FIGS. 1A to 1D are cross-sectional views illustrating a conventional method of forming a flash memory device.
Referring to FIG. 1A, a plurality of memory cells Cell and select transistors Sel Tr are formed in a cell region of a semiconductor substrate 10 through conventional processes. A transistor Tr is formed in a peripheral region of the semiconductor substrate 10. An insulating layer 11 is formed over the entire surface of the semiconductor substrate 10 including the peripheral and cell regions thereof. A drain contact hole 12 in a drain region between the select transistors Sel Tr is formed by an etch process. The drain contact hole 12 is filled with a polysilicon layer 13a. 
Referring to FIG. 1B, a Chemical Mechanical Polishing (CMP) process is performed on the polysilicon layer 13a to expose the top surface of the insulating layer 11. Thus, a drain contact 13 is formed.
Referring to FIG. 1C, an etch process is performed in such a manner that a junction region adjacent to the transistor Tr in the peripheral region is exposed, thus forming metal contact holes 14. A tungsten layer 15a is formed to fill the metal contact holes 14.
Referring to FIG. 1D, a CMP process is performed on the tungsten layer 15a so that the top surface of the insulating layer 11 is exposed. Therefore, the metal contacts 15 are formed.
In the above method, a seam is formed on the top surface of the drain contact 13. The seam is weakened during a cleaning process subsequent to the CMP process performed on the polysilicon layer 13a. Further, during the deposition of the tungsten layer 15a, tungsten infiltrates the seam area, causing a variation in the contact resistance value.
The above method also requires separate etch, deposition, and CMP steps for each of the drain contact 13 in the cell region and the metal contacts 15 in the peripheral region. Further, two CMP processes must be performed even though the final polishing height resulting from the CMP process for forming the drain contact 13 is the same as that of the CMP process for forming the metal contacts 15. Accordingly, the process time is lengthened and the production cost is increased.